1. Field of Invention
Extraction and use of a unique resource of dissolved natural gas in Lake Kivu (Rwanda-DR Congo) deepwater, addressing especially problems and opportunities of post-degassing water flow and reinjection, and overall engaging five factors of optimization: (i) lake safety; (ii) productive utilization of additional deepwater resources; (iii) methane extractive efficiency, including tapping additional resource zones; (iv) CO2 degassing and capture with high efficiency power production; and (v) stewardship of the lake's extraordinary beauty and ecosystem.
2. Description of Related Art
Demand for electrical power is expanding rapidly in the region of Lake Kivu. As untapped hydropower resources are limited, rapidly increasing electric power demand has been supplied by banks of diesel engine generators consuming fuel imported from the East African coast with power costs sometimes exceeding US$0.44/kWh. This trend burdens local economies. It is a pressing matter, for example, in the case of Rwanda's fast-growing economy, where a Government agenda has been to expand power generation capacity by close to a factor of four in two years' time remaining to meet a longstanding 2017 goal. Such high cost power is unattractive to the kinds of new businesses required to boost Rwanda into higher levels of industrializing business activity needed for long-term growth in per capita productivity and income.
Lake Kivu contains an estimated 32 million tonnes (approximately 1.5 trillion cubic feet, TCF) of natural gas trapped in convectively isolated deepwater below 260 meters. This bottom zone is called the “Main Resource Zone” (MRZ). (Many scientific perspectives on Lake Kivu are presented in the papers collected together in a comprehensive 2012 book edited by Descy et al.) The trapped gas resource in the MRZ has the capability to provide approximately 800 megawatts continuous power over a period of 40 years if extracted and combusted with high efficiency. An additional amount of natural gas, roughly 6 million tonnes, is trapped in another, higher level, convectively isolated, anoxic layer situated between ˜260 and ˜200 meters depth. This layer is called the “Potential Resource Zone” (PRZ). Even more methane, approximately 8.6 million tonnes, is trapped above the PRZ in a higher convectively isolated anoxic layer called the “Intermediate Zone” (IZ) in the depth interval ˜200 m to ˜80 m, just below the convectively oxygenated “biozone” (BZ) existing above ˜80 m depth, (Wuest et al., 2009).
Many problems are involved in the extraction and utilization of Lake Kivu methane and in the replacement into Lake Kivu of return flows of deepwater that have been extracted for degassing at or near the surface. No method or system has been described or practiced that could access methane in the PRZ and/or IZ in an efficient manner. Nor has any method or system been described or practiced that could make use of abundant useful additional resources present in Lake Kivu deepwater in connection with an overall system and method for extracting and utilizing the lake's deepwater methane reserves. Nor has any method or system been described or practiced that in doing so substantially increases the safety of Lake Kivu against a mass catastrophe release of the CO2 trapped within its depths triggered by lakebottom volcanism of the type already known to have occurred with catastrophic effect in the past, most recently about 900 years ago. All such goals are important. No closely related art exists pertinent to accomplishing them, with one exception.
In US 2015/0354451 A1, the inventor of the present invention disclosed a method and system for the combination and integration of a Total Degassing System (TDS), an OxyFuel Combustion Power System (OXFCPS), and a Return Flow System (RFS) applied to a type of lake of which Lake Kivu is the only known example of commercially significant size. The RFS is described in a general sense, but there remains room in the art for further development of the method and system and particularly the RFS aspect of same.
A multi-stage CO2 removal method for gas extraction and cleaning has been in use on Lake Kivu since the 1950s with various modifications to the present. In a 2-stage method, the first stage is differential degassing of CH4 and CO2 in upwelling deepwater constrained in an upflow pipe and/or chamber. Deepwater gas typically is degassed at a depth of approximately 20 meters. This depth favors CH4 degassing (˜80%) over CO2 degassing (˜20%). The next stage is bubbling flow of degassed gas obtained from stage-1 up through a “wash” of biozone water at a near surface pressure. This step further cleans the gas. It redissolves a high fraction of CO2 with loss of only a small fraction of CH4, typically less than 10%, (but depending upon the volume ratio of gas to the washing water it is exposed to factored also by the bubble size distribution and bubble passage exposure time). The process is illustrated helpfully on the web site of Michel Halbwachs (Halbwachs, web site). It was elegantly designed by Belgian engineers in the 1950s for its initial purpose to extract combustible methane from the lake using only water-based processes to remove the very large component of associated CO2. The Belgian design was not created to optimize use of the total stored resource present in the lake. Nor was it developed in view of danger from catastrophic limnic eruption. The phenomenon of limnic eruption was only understood as a consequence of extensive international research funded in the aftermath of deadly eruptions of two small volcanic lakes in Cameroon in the 1980s. The Belgian method also cannot be used to extract methane from Lake Kivu's PRZ or IZ. Methane concentrations in these layers are too low.
The standard technology practiced on Lake Kivu for deepwater methane extraction injects acidic CO2-rich scrubbing water into the biozone. The term scrubbing water corresponds to the flow of near surface water used in gas scrubbing bubble flow gas exchange processes. Scrubbing by bubble flow occurs in tower structures. The interaction of extracted gas with near surface water absorbs CO2 into solution as a means of scrubbing CO2 out of stream of methane-containing gas that typically is degassed at a depth of about 20 meters depth in two-stage degassing systems such as the KP-1 facility on Lake Kivu.
The presence of an estimated ˜600 million tonnes of associated CO2 trapped at depth in Lake Kivu in the MRZ and PRZ presents a problematic efficiency challenge for the extraction of conventionally combustible methane. Methane cannot be combusted efficiently in the presence of large amounts of CO2 if conventional methods are used. Pre-combustion separation of CO2 by various technologies requires substantial power loss as well as methane loss or “slip,” as well as large capital investment in equipment.
Oxyfuel combustion is applicable to power production from methane in turbines (Jerica and Fesharski, 1995; Clean Energy Systems, 2006; Hammer et al., 2009; Revzani et al., 2009; Woolat and Franco, 2009). The inventor in US 2015/0354451 has disclosed a method and system that shows how oxyfuel combustion allows Lake Kivu gas operations to be technologically upgraded in a way that can solve core aspects of the basic problems described herein. Oxyfuel combustion offers a way to utilize Lake Kivu deepwater gas without need for CO2 separation in the extraction process. This allows dissolved gas to be totally extracted from Lake Kivu deepwater including both methane and CO2, (Hussain 2001).
The presence of so much CO2 trapped at depth in Lake Kivu presents a threat of mass asphyxiation mortality from a possible convective runaway “limnic eruption.” An event could be triggered by lake bottom volcanism in the northern sector of the lake where bathymetric surveys have revealed the presence of several volcanogenic cones on the deep lake bottom. Again, scientific studies of sediment cores from Lake Kivu have revealed evidence of convective runaway events in the past. These appear to have been triggered by volcanic activity according to the evidence found in these cores (Ross, 2013a, b; Ross et al., 2014, 2015; Hecky and Reinthal, 2010). Lake Kivu is a rift-structured lake. It represents the beginnings of the formation of a possible new ocean. Lake Kivu sits within the western “Albertine” rift of the East African rift system. Its existence is directly associated with the most active volcanic province in Africa. Lake Kivu is bordered directly to the north by two active volcanoes, Nyiragongo and Nyamuragira. To the northeast is the Virunga chain of volcanoes, at least one of which has been active within the past 100 years. The area of the northern lakeshore is littered with volcanic cones as is the lakebottom in the northwestern sector. The periodic emplacement of rift-oriented vertical sheet-like dykes of magma under Lake Kivu is a generic expectation of its embryonic tectonic rifting situation. The tectonics of Lake Kivu combines aspects of continental rifting, new ocean crust formation and Iceland-like plume province interactions with rifting. In non-technical parlance, this is to say that volcanic activity on the lake bottom is to be expected in the future, continuing what is shown from the bathymetric record: that eruptive activity has been frequent over the past ˜10,000 years.
By the end of 2014, only one small test power station was producing power from Lake Kivu deepwater gas on a scale ranging from 1 to 3 megawatts. This station utilizes a two-stage extraction & CO2-removal gas cleaning process. The basic system design was a modification of engineering invented in the 1950s by Belgian scientists and engineers. The power output performance of this test station, “KP-1,” was well below its design specification. The project suffered from long-term disputes including litigation as a consequence of the performance shortfall and other matters. The fraction of power consumption by gas extraction operations is not disclosed publicly but is widely expected to be substantial.
A stage of extensive research by experts and associated deliberation over extraction policies was finalized in 2009 with publication of Lake Kivu Management Prescriptions (LKMP, 2010) and an associated paper by Wuest et al., (2009): “Modeling the reinjection of deep-water after methane extraction in Lake Kivu.” The deliberative process was thoroughgoing and well informed. It included an impressive quality and quantity of scientific data, insights and analyses (e.g., Tietze 2007, 1981, 1978; Schmid et al., 2003, 2005; Pasche et al., 2009; Wuest et al., 2009, 2012; Descy et al., 2012). However, it lacked understanding of the causes and chronology of lake convective events later illuminated by the research of Kelly Ann Ross and colleagues (Ross, 2013a, b: Ross et al., 2014, 2015). Despite the very high quality of input data and analysis, the conclusions made in the 2009 Management Prescriptions were, however, fraught with disagreement over unavoidable conflicts over conflicting agendas of long-term safety, process efficiency and ecological stewardship involving CO2. The most senior scientist involved disassociated himself from the final document, principally over concern for long-term human safety. This scientist's main concern was that CO2 must be removed (co-degassed with methane) from the gas-rich deep layer rather than returned in large fraction in return flow reinjection into the deep lake. Technologically, degassing CO2 from the return flow was not in principle a difficult design change issue. Such a change, however, would vector Lake Kivu's massive CO2 reservoir to be vented to the atmosphere. Return of approximately 80% of the concentration of CO2 into the deepwater was generic to the technology of the time.
In the perspective of hindsight, four significant aspects may be noted about the deliberations and disputes over the Lake Kivu Management Prescriptions and the associated technical advisory report of Wuest et al., (2009) concerning return flow reinjection scenarios and recommendations. First, the situation was pressured by significant commercialization agendas involving at least three different ventures, all seeking to implement variations upon the well-proven, Belgian technology inaugurated in the 1950, and all responding to a pressing need to produce electricity in the region such as was recognized and emphasized by all parties including the two host governments involved. Second, no new technology options were then in view such as would have been able to illuminate ways to overcome basic constraints of the old Belgian technology. Third, no understanding then existed on how potentially to utilize, rather than simply vent, degassed CO2 in very large amounts amounting to millions of tonnes per year. Fourth, no understanding then existed: (i) that past catastrophic degassings of Lake Kivu occurred; and (ii) that past degassing events appear not to have been spontaneous (that is, occurring when deepwater gas build-up reached a criticality point with respect to ordinary perturbations) but rather triggered by volcanic events.
The present day density structure of Lake Kivu is robustly stable against convective runaway (Kling et al., 1989; Wuest et al., 2009; Schmid et al., 2003, 2005, Lorke et al., 2004). There is no present day risk of a convective runaway arising spontaneously or as a consequence of ordinary perturbations such as, for example, sinking of a heavy object into gas-saturated sediments at 300 meters depth. Recent research findings based on sediment cores analysis from Lake Kivu, however, suggest there have been at least two large scale gas release runaway events occurring in Lake Kivu over the last 4,000 years (Ross, 2013a,b; Hecky and Reinthal, 2010). Both are associated directly in the sedimentology with evidence of initiating eruptive activity. The last event was approximately 900 years ago. A reasonable though not certain conclusion based on bathymetric evidence is that convective gas release events in Lake Kivu have been in the past, and in the future can be, triggered by sublacustrine volcanic events. Such events may be all-at-once runaways. Or they may extend over periods of time in local plume convection zone. A convective plume over an active volcanic sublacustrine eruption cone would pump local convective degassing. In certain conditions, this would not spread a self-sustaining convective front. It would limit degassing to the surface to a single plume zone of convection pumped from below by volcanogenic gas, pumice and heat energy release. Such point-like convective degassing would have the potential to degas fractions of Lake Kivu's deepwater gas reserve over various periods of time. Detailed multivariate computational modeling would be required to explore a range of estimates.
Sublacustrine volcanism differs from subaerial volcanism in that it does not involve associated sub-aerial degassing. Deep sublacustrine eruptions release significant gas flux upwards that is dissolved into the water into which it is released. In Lake Kivu, such conditions will increase dissolved CO2 levels locally above the gas input zone thereby approaching a condition of saturation and consequent formation of bubble-driven buoyant ascending plume dynamics. This is especially so for Lake Kivu because the local eruptive magma types are CO2-rich. Sublacustrine events in Lake Kivu can involve rapid point-like release of possibly massive amounts of magmatically-derived CO2 along with hot magma and biologically derivative gases accumulated in large quantities to bubble-forming saturation levels in deep sediments. Deepwater sublacustrine volcanism in Lake Kivu therefore can cause plume-like, sometimes explosive, fountaining of multi-phase plumes penetrating from the lake bottom to various depth levels in the lake, including above the lake's surface. Such events release massive amounts of heat, saturated bubble-forming gases, especially CO2, bubble-rich heated muds, and rising columns of buoyant bubble-rich magmatic pumice. Analogs to such events are well known from oceanographic study of a variety of types of subsea volcanos. Were such a deepwater eruptive event to take place today in Lake Kivu, it is possible that it would nucleate runaway convection in the lake. Were this to happen, a large fraction of the lake's store of ˜600 million tonnes of trapped CO2 and ˜50 million tonnes of trapped methane would be released catastrophically. A slower “plume-pumped” event extended over time also could be dangerous as well. It could generate an asphyxiating layer of CO2 locally. Apart from human safety matters, the loss of Lake Kivu's gas resource would be a gigantic economic catastrophe from the loss of its electric power potential and other substantial adjunct sources of value.
Satellite interferometric geophysical evidence is clear that the 2002 Nyiragongo eruption within the city of Goma through rift-oriented cracks was associated with deep rift-axial near-vertical faulting and magma dike emplacement extending for several kilometers under the lake in the northern sector (Wautier et al., 2012). This geophysical observation that perhaps only slight differences in the geodynamics of subsurface magma dyke emplacement in 2002 could have generated a sublacustrine eruption at depth in the range 450 to 300 meters. It is conceivable therefore that a convective runaway event could have been triggered at that time. Had that happened, the possible death toll from asphyxiation could have exceeded that of the entire Rwandan genocide of 1994.
The risk of such a future event can only be reduced substantially by two related processes. These are: (i) substantial reduction of CH4 and CO2 concentrations present in the MRZ, and (ii) deflation of the volume extent of the MRZ with consequent lowering of the depth position of its upper boundary known as the main density discontinuity (MDD). This discontinuity is presently located at about 260 meters depth. The MDD is slowly rising over time. This is due to natural inputs of CO2-rich mineral waters from deep springs in the northern sector adjacent to the Nyiragongo volcanic field on land and its sublacustrine extension. Human action to reverse this process and deflate the MRZ by utilizing its resources is highly prudent. An accelerated timescale would be wise.
Because of its extraordinary beauty, it can seem reasonable to presume Lake Kivu to be a harmonious ecosystem. This would be an error. Lake Kivu is naturally unstable. It is subject to a slow irregular cyclic process producing occasional catastrophic events. This cycle involves slow build-up of dissolved gasses within a slowly inflating deep layer of dense water acting as a gas trap. Lake Kivu's dense deepwater comes from deep springs that release water rich in dissolved CO2 and in Na-, K-, Mg- and Ca-bicarbonates generated by volcanic-hydrothermal circulations with associated weathering of large volumes of young igneous rock. Fluxes of geogenic CO2 and biogenic CH4 become trapped in the deepwater layer over time. Extended phases of slow build-up of trapped gas end with catastrophic degassing. Sublacustrine volcanism of sufficient intensity will trigger runaway convection in circumstances whenever a sufficiently large layer of dense deepwater is present containing high enough levels of trapped gases to sustain large-scale convective dynamics once triggered. Otherwise a sublacustrine eruption may power convection during an extended period of plume-pumping above one or more sublacustrine sites of active volcanism.
Catastrophic lake mixing and degassing events cause local extinctions of oxygenic life both within the lake and on land peripherally within the Lake Kivu basin. These events are ecological catastrophes. They generate mass scale admixture of concentrated deep nutrients into surface waters. This creates eutrophication from algal megablooms and die-offs that cause bacterial de-oxygenation of the water column. Sediment core records show this clearly. Continuation of this natural cycle into the future conflicts with both human safety and ecological stewardship.
Lake Kivu's gas-rich deep layers have been accumulating gas for almost a thousand years to a presently dangerous level. An unavoidable conclusion from such considerations is that the most gas-rich gas-trapping deep layer should be substantially degassed. Carefully controlled artificial removal of the deep layer is a way to change and stabilize the natural process of repeated catastrophe. Such a removal process can both preserve the lake's ecosystem and provide a major driver for development.
Differences between the total mass of solutes in water and a density effect from their presence in solution are due to “haline contraction” effects (Monin, C., 1994; Appelo, 2014; Imboden and Wuest, 1995; Wuest et al., 1996; Millero, 2000; Boehrer and Schultze, 2008, 2009; Thierry et al., 2014; Schmid et al., 2003). For example, Lake Kivu MRZ deepwater contains total dissolved solids (TDS) of ˜6 gTDS/l, whereas the in situ density effect contributed by these dissolved solids is ˜⅔rds as large: ˜4 g/l. Lake Kivu MRZ deepwater also contains up to almost 4 grams per liter of dissolved CO2. However, this amount of dissolved CO2 in the MRZ contributes to an in situ excess water density of only ˜1.1 gram per liter. Methane does not contribute excess density. In the MRZ of Lake Kivu it contributes a small negative density effect. PRZ water directly above the main discontinuity contains TDS at a level of roughly half of the MRZ amount: ˜3 gTDS/l. The excess density effect in this water is (at ˜⅔rds of the TDS): ˜2 g/liter. The density discontinuity at ˜260 meters depth a strong density contrast: ˜2 g/l. This acts as a lid stabilizing and protecting the situation of Lake Kivu against upwards convention of the gas-rich dense deepwater. Therefore removal of CO2 from MRZ water leaves it too de-densified to be reinjectable into the MRZ and too dense to be reinjectable into the PRZ with preservation of horizontal layering stability.
Dissolved solids present in Lake Kivu represent potent and abundant bio-nutrients. The main component in terms of mass is bicarbonate ion, HCO3−. Bicarbonate ion represents on average roughly 4 grams per liter of TDS in MRZ water. This concentration of bicarbonate ion corresponds to ˜0.8 grams of carbon per liter (˜67 mM/l).
In Lake Kivu, the alkaline earths magnesium (Mg) and calcium (Ca), complex with bicarbonate and carbonate anions as doubly charged cations. These are present in MRZ deepwater at concentration levels of approximately 350 mg/l, (15 mM/l), and 150 mg/l, (3.7 mM/l), respectively. The alkali metals sodium (Na) and potassium (K), which also complex with bicarbonate and carbonate anions, but as singly charged cations, are present in MRZ deepwater at concentration levels of approximately 490 mg/l, (21.3 mM/l), and 320 mg/l, (8.18 mM/l), respectively (Tassi et al., 2009).
Bicarbonate is used by certain algal species, both prokaryotes and eukaryotes, as a main carbon source. Bicarbonate utilization systems in algae capable of bicarbonate utilization acts via a carbon concentrating mechanism (CCM) providing CO2 to RuBisCO in the carbon-fixing photosynthetic process. This mechanism exhibits an overall chemical process formula HCO3−⇄CO2+OH−, (Azov, 1982; Shiraiwa et al. 1993; Talling, 2010; Chi et al., 2011; Chen et al. 2011; Giordano et al., 2005; van Hille et al., 2014). Carbon fixation from bicarbonate consequently causes a pH rise in the host solution. This is a widely observed and well-demonstrated natural phenomenon seen in many algal blooms as well as in laboratory tests (e.g., Talling, 1976, 1985, 2010; Wurts and Durborow, 1992; Eckert and Hambright, 1996; Uusitalo, 1996; Tucker and D'Abramo, 2008; Cerco et al., 2013; Keymer et al., 2014). pH-raising hydroxyl (OH−) addition into solution corresponds directly on a mole-per-mole basis to photosynthetic carbon fixation into algal biomass.
Once degassed of CO2, the chemistry of Lake Kivu MRZ 375 m deepwater results is a Na—K—Mg-rich alkaline water with 4.2 grams per liter (69 mM/l) bicarbonate ion. The pH of this water when CO2-rich in situ at 375 meters depth is 6.15 (Tassi et al., 2009). When fully degassed, such water, when in equilibrium with atmospheric CO2 at sea level, will have pH ˜10.1, according to the theoretical relation, pH=11.3+log(HCO3-), and empirical checkpoints of Wright, (1983) and Psenner and Catalan, (1994, see also Talling, 2010), where the bicarbonate ion concentration is input in units of moles per liter. A higher pH can be reached by further CO2 removal beyond that which corresponds to equilibrium with atmospheric CO2.
In the carbonate system in fresh water, bicarbonate ion is the dominant carbon-carrying molecule at this pH in Lake Kivu (Tassi et al., 2009; Mook. 2000): ˜97% HCO3− and ˜3% CO32−. The most abundant bicarbonate-complexing cation in Lake Kivu deepwater is magnesium: 0.38 g/l (15.6 mM/l) at 375 meters depth (Tassi et al., 2009). Of the total 4.2 gTDS/l bicarbonate component in 375 m deepwater, the Mg2+-complexed bicarbonate component is 2.3 TDSg/l or 55%, whereas the Ca2+-complexed bicarbonate component is 0.5 TDSg/l or 12%. Additional bionutrients present in Lake Kivu MRZ deepwater (Tassi et al., 2009) include the basic “NPK” bionutrients: ammonium ion (NH4+: 55 mg/l, 3.2 mM bio-available Nil), phosphorus (P: ˜5 mg/l, 0.16 mM/l) and potassium (K: ˜320 mg/l, ˜8.2 mM/l), as well as silicon (Si) and sulphur (S), plus other trace biologically significant trace elements such as Fe, Zn, Se and Mo.
pH-dependent magnesium hydroxide precipitation is well known to provide a basis for autoflocculation of algal biocultures when sufficient magnesium is present in bioculture solutions (Golueke and Oswald, 1965; Elmaleh et al., 1991; Tesson et al., 2008; Spilling et al. 2010; Vandamme et al., 2012, 2013, 2014, 2015; Vandamme, 2013; Wu et al., 2012; Smith and Davis, 2012; Gonzalez-Fernandez and Ballesteros 2012; Besson and Giraud, 2014, Garcia-Perez et al., 2014; Baya et al., 2014; Choi, 2014). Magnesium concentrations of 5, 10 and 15 mM/l have been demonstrated to be highly effective for flocculation (Garcia-Perez et al., 2014). In the experiments of Garcia-Perez et al., (2014), using an Mg concentration of 15 mM/l, flocculation efficiency is 98% effective at pH 10.5 with 18% of Mg precipitated, whereas 97% percent of Mg is precipitated by pH 11.0. Based upon other observations, Mg would be expected begin to precipitate at about pH 9.7 in extracted Lake Kivu deepwater obtained at 375 meters depth (O'Connor et al., 2009; Langmuir, 1997; Abrams et al., 1999). By pH 10.4, Mg solubility would have decreased to ˜20 mg/l such that, 95% of the originally present Mg in solution will have precipitated. These data sets differ. In general, trace levels of CO2 in solution are likely to influence all experimental results. Also in carbonate-dominated chemistries, varying CO2 levels in solution will govern varying pH and varying degrees of magnesium hydroxide precipitation. Lake Kivu deepwater that is extracted and degassed covers a huge range in CO2 content in such a process, beginning with up to almost 4 grams CO2 per liter dissolved in solution. The dissolved CO2 amount controls the pH in this water type (Ying et al., 2014). Calcium has lower solubility than magnesium and is observed to co-precipitate with magnesium hydroxide in cases of rising pH precipitating large quantities of magnesium hydroxide, for example in flocculation processes (Vandamme, 2013).
Efficient techniques have been developed for surface skimming of flocculated algae that has been separated to the surface of bioculture by microbubbles attachment. Microbubble supply may be by conventional Dissolved Air Floatation (DAF) methods (Aulenbach et al., 2010), or by methods utilizing new fluidic oscillation technologies for microbubble production (Zimmerman, 2011; Zimmerman et al. 2011; Hanotu et al., 2012, 2013). DAF separations are a long-established industrial technology.
Wet magnesium hydroxide with admixed calcium carbonate is an input material in the production of H2O— and CO2-absorbing “greentech” cements and precast pozzolanic concretes and related building materials such as wall and paving bricks, cinder blocks, roof tiles and reinforced columns and beams (Harrison, J: TecEco website; Vandperre and Al-Tabbaa, 2007; Liska and Al-Tabbaa, 2007, 2008, 2009; Liska et al., 2008; Unluer and Al-Tabbaa, 2013a,b, 2014a,b).
Algal biocultures operated in high pH alkaline situations offer special advantages for “greentech” algal-bacterial biomass production operations. They allow efficient, low waste, input of CO2 into open air or otherwise non-sealed biocultures (Chi et al., 2011, 2013, 2014). A first advantage is that CO2 can be injected for “CO2 loading,” that is to reduce carbonate ion concentrations by recharging bicarbonate concentrations. In the high pH range, CO2 absorbs readily into solution in the net of gas exchange with the atmosphere. A second advantage of pH-swing recharge cycle is that it can be used to balance diurnal (diel: day/night) cyclicality in photosynthetic carbon fixation. Photosynthesis can boost pH during the day by carbon biofixation. When photosynthesis is not active during the night, continued CO2 addition can be absorbed by chemical absorption into solution, decreasing carbonate ion abundance and increasing bicarbonate ion abundance.
Previous disclosed teachings help to understand: (i) how nutrient-rich deepwater can be to grow marine diatoms used in aquaculture (1983: U.S. Pat. No. 4,394,846); (ii) how water produced from drilled wells can be can be treated to remove pollutants (2009: EP 2 093 197 B1); (iii) how wastewater can be reclaimed by algal production (1973: U.S. Pat. No. 3,780,471; 2012: U.S. Pat. No. 8,101,080 B2); (iv) how bicarbonate can be an important aspect of high-yield lipid production in algal bioculture production (2013: US 2013/0295623 A1); (v) how algal increase in pH by protosynthesis effects changes in carbonate-bicarbonate equilibria that allow efficient diurnal CO2 capture into algal production systems (2013: US 2013/0319059); (vi) how potassium can be recovered from waste waters by algae (2012: US 2012/0061315 A1). There are long-established teachings on mass algal culture using waste water (for example: 1965: 3,195,271). There also are recent disclosed teachings on algal bioculturing in batch mode in photobioreactor (for example: US 2010/0112649 A1). Also there are recent teachings on treatment of produced waters from fracking operations (for example: US 2014/0076817 A1, and US2012/0175308 A1). (For a recent review on treatment of produced water, see: Duraisamy et al., 2013.) While various of these teachings provide helpful insight related in various ways to the specific method disclosed herein, none provide specific insight for addressing the specific chemistry, challenges, constraints and opportunities of Lake Kivu.
Accordingly, there is a pressing need and substantial opportunity for a new method and system to extract Lake Kivu deepwater in a way that degasses CO2, ensures lake safety over time, utilizes resources other than methane, produces products other than electric power, accesses methane otherwise untapped by conventional technologies, couples degassing with hyper-efficient oxyfuel combustion turbine and CO2 power cycle technology, and couples CO2 degassing into various possibilities of CO2 utilization.
All references cited herein are incorporated herein by reference in their entireties.